Known water heating systems for divers working from electric-powered submarines use a closed hot-water circuit in which the water is heated by electrically-resistive heating elements. The resistive heating elements are heated by an electric current supplied from batteries carried in the submarine. A considerable portion of the total energy carried in the ship can thereby be required for heating. For example, for the heating of a diver and a water-filled diving chamber which provides the diver with an enclosed base of operation, a heat output of roughly 3.5 kW is required in water 300 m deep at a water temperature of +4.degree. C. The ship's batteries, however, typically must also supply a propulsion plant, a working floodlight, electronic equipment, and underwater tools with energy. In typical work submarines having a diving hatch and designed to carry a crew of two, the usual charge of energy stored in the batteries is on the order of about 30 to 60 kWh, about half of which is required for propulsion and maneuvering.
The duty time of such a battery-powered submarine is limited by its store of energy. To maximize the duty time it is necessary to operate the ship as energy efficiently as possible. Nonetheless, diving chambers and divers must be adequately supplied with heat in order to obtain sufficient working time for the divers at tolerable working conditions. The problem of heating is particularly important in deep-sea diving, because deep-sea divers breath a helium-oxygen mixture and the helium portion of the respiratory mixture draws approximately seven times the amount of heat from inside the body as air does at the same temperature. This internal loss of heat together with the external loss of heat from the diver's body to the surrounding water through the thermal diving suit must be counterbalanced. The 3.5 kW heating value mentioned above is a typical value for the total internal and external heat loss of a deep-sea diver.
One of two approaches is generally adopted with respect to supplying energy for conventional resistance-heated hot-water circulation systems in present submarines adapted to support an external deep-sea diver, e.g. submarines with diving hatches and hot-water circulation systems. Generally either no provision is made for additional energy for the hot-water system, in which case the duty time of the submarine is significantly reduced, or additional energy is provided by increasing the number of storage batteries. However, increasing the number of batteries to avoid reducing the duty time requires increasing the size of the submarine significantly, thereby reducing the maneuverability of the ship. Increasing the size of the submarine also leads to an increase in the propulsive power required, which leads to a further increase in the battery capacity required. Thus serious drawbacks attend both approaches.